Conventionally, it is well-known a gas substitution apparatus in which an inside of a retainer body is filled with gas (for example, oxygen), and is pressurized, thereafter liquid (for example, water) flows downward within the inside of the retainer body, thereby gas dissolved in liquid from a beginning (gas existing in the liquid from a beginning under atmospheric pressure, for example, nitrogen) is substituted by gas filling the inside of the retainer body (for example, oxygen) (that is to say, for example, gas dissolved in water is substituted from nitrogen by oxygen).
For example, in Patent Literature 1, a gas substitution apparatus is disclosed in which liquid (for example, water) is discharged toward upper direction in the inside of the retainer body filled with gas (for example, oxygen) and liquid flows downward along a rectification board arranged in the retainer body. In this gas substitution apparatus, liquid flows downward along the rectification board, thereby gas dissolved in liquid is substituted by gas (for example, oxygen) filling the retainer body.